CHARACTERIZATION OF HYDROGENATED AMORPHOUS-SILICON FILMS BY A PULSED POSITRON BEAM

A low-energy pulsed positron beam was, for the first time, applied to the characterization of hydrogenated amorphous silicon films (approximately 1-mu-m in thickness) deposited by means of plasma-enhanced chemical-vapor deposition. By the use of a pulsed positron beam, positron lifetime spectra were measured on four films deposited under different rf-power densities. In two of the films prepared at the intermediate power densities, a long-lived component (tau congruent-to 9 ns) was observed, which indicates that a high concentration of voids exists. A component with a lifetime of 319 ps, which is due to trapped positrons at divacancies or small vacancy clusters, was observed in the film prepared at the lowest power density, while a component with a lifetime of 390 ps-440 ps, which is due to trapped positrons at large vacancy-type defects, was observed in the other films. The results clearly indicate that the defect properties are strongly influenced by the rf-power density.